By Jake Bumpus & Alan O’Donovan, Safety Engineering Consultants with Occupli Consultancy
Due to the requirements of the Machinery Directive, many machines are installed with some form of emergency stop. This is commonly in the form of a push button, but they can also be wires, ropes, bars, handles or foot-pedals. The general principles of design of emergency stops (E-stops) are described in the harmonised standard, EN ISO 13850:2015.
According to EN ISO 13850, there are two categories of emergency stops. The category of emergency stop used in a machine should be based upon the risk assessment carried out by the manufacturer during the design phase, in accordance with the principles in EN ISO 12100.
- Category 0 E-stops operate on the principal of “Stopping by immediate removal of power to the machine’s actuators”
- Category 1 E-stops work by “Stopping movements and operations with power available to the machine actuators to achieve the stop and then removal of power when the stop is achieved”.
Some of the key requirements of the Emergency Stop function according to ISO 13850 are discussed below.
Functional Requirements
The emergency stop function should be a complementary protective measure only (i.e. the last line of defence for emergency scenarios), and not a substitute for safeguarding measures and other safety functions.
Once activated, an emergency stop device should be designed to remain activated until the actuator has been disengaged. This should only be possible by an intentional action – most commonly with push buttons, this would take the form of a twist to release. It is also important that disengaging the emergency stop does not automatically restart the machine. Restarting the machine should require an additional start command.
Determination of the performance level (PL) or SIL required by the emergency stop safety function should be determined using the methodology of ISO 13849-1 or EN 62061, but the minimum required in all cases is PLr c or SIL 1.
Colour & Labelling
The actuator of an emergency stop device should be red, and the background colour should be yellow.
The most recent revision of the standard also recommends that neither the actuator nor the background should be labelled with text or symbols. Where a symbol is needed for clarification, the symbol from IEC 60417-5638 should be used:
Location & Positioning
The location of any emergency stop devices should be determined by the risk assessment. Commonly, they will be required at each operator control station, and also at locations where intervention to the machinery is needed.
For emergency stop devices that are actuated by hand (e.g. push buttons) they should be placed in a position where they can be easily activated by the palm of the hand, and should be mounted between 0.6 m and 1.7 m above the access level (e.g. floor or platform level).
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